Prosthetic devices employing oxidized zirconium and other abrasion resistant surfaces contacting surfaces of cross-linked polyethylene

ABSTRACT

Orthopedic implants which includes the components of zirconium or zirconium-based alloys having surfaces coated with oxidized zirconium or alternatively, other orthopedic implants comprising abrasion resistant surfaces contacting surfaces of cross-linked polyethylene are disclosed. Such implants provide low friction, highly wear resistant coatings especially useful in artificial joints, such as hip joints, knee joints, elbows, etc., but also useful in other implant devices as well.

BACKGROUND OF THE INVENTION

[0001] This invention relates to metallic implants with load bearingsurfaces coated with a thin, dense, low friction, highly wear-resistant,uniformly thick coating of oxidized zirconium. This invention alsorelates generally to metallic implants with load bearing, abrasionresistant surfaces. In the present invention, the load bearing oxidizedzirconium surfaces or abrasion resistant surfaces contact counterbearing surfaces of cross-linked polyethylene (XLPE). XLPE has superiorwear characteristics compared with other conventional polymer materialsused in prostheses. Oxidized zirconium has thermal conductivitycharacteristics that are particularly advantageous when used in aprosthetic device in which it articulates against XLPE. The uniqueadvantages of oxidized zirconium and abrasion resistant surfaces incombination with those of XLPE result in a synergy which allows one toaccentuate the superior properties of XLPE as a counter bearing surface,resulting in a superior prosthetic device.

[0002] Historically prostheses of articulating surfaces were constructedof materials of differing hardness for the contacting surfaces. Byhaving one “yielding” surface, such prior art devices eventually form anoptimal fit, i.e., a tight tolerance, whereby galling, fretting, andother erosive phenomena are minimized, resulting in longer-lastingprosthetic devices. An example of these early-generation devices is thefemoral head of a hip-stem prosthesis which engages a counter-bearingsurface in an acetabular cup which is often made of a softer materialsuch as ultra-high molecular weight polyethylene. However, use ofcontacting surfaces of different hardness is not a perfect solution. Thesofter surface is, by nature sacrificial; it will eventually fail, itsmain virtue is the realization of an overall increase in the useful lifeof the prostheses. Additionally, fretting of the softer surface resultsin debris that may have deleterious effects on the health on thepatient.

[0003] The invention described herein is a particular type ofceramic-on-polymer prosthesis. Its unique compositional propertiesaffords the traditional advantages of ceramic-on-polymer systems whileavoiding their major disadvantage.

[0004] The invention overcomes the major disadvantage generally inherentin prosthetic devices having hard surfaces articulating against softsurfaces. The basic technology upon which the improvement describedherein is based, is described in U.S. Pat. No. 5,037,438 to Davidson andto commonly assigned, copending application Ser. No. 09/381,217, filedNov. 24, 1999 of Hunter, et al., both of which are fully incorporated byreference herein.

[0005] The longevity of medical implant devices is of prime importanceas it is desirable that the implant should function for the completelifetime of a patient. This is particularly true if the patient is youngand the number of surgical revisions is to be kept to a minimum andpreferably zero. To this end, orthopedic implant materials shouldpreferably combine high strength, corrosion resistance and tissuecompatibility. One of the variables affecting the longevity ofload-bearing implants such as hip-joint implants is the rate of wear ofthe articulating surfaces and long-term effects of metal ion release. Atypical hip-joint prosthesis includes a stem, a femoral head and anacetabular cup against which the femoral head articulates. Wear ofeither or both of the articulating surfaces results in an increasinglevel of wear particulates and “play” between the femoral head and thecup against which it articulates. Wear debris can contribute to adversetissue reaction leading to bone resorption, and ultimately the jointmust be replaced.

[0006] The rate of wear of the acetabular cup and the femoral headsurfaces of artificial hips is dependent upon a number of factors whichinclude the relative hardness and surface finish of the materials whichconstitute the femoral head and the acetabular cup, the frictionalcoefficient between the materials of the cup and head, the load appliedand the stresses generated at the articulating surfaces. The most commonmaterial combinations currently used in the fabrication of hip-jointimplants include femoral heads of cobalt, titanium, or zirconium alloysarticulating against acetabular cups lined with organic polymers orcomposites of such polymers including, for instance, ultra-highmolecular weight polyethylene (UHMWPE) and femoral heads of polishedalumina in combination with acetabular cups lined with an organicpolymer or composite or made of polished alumina.

[0007] Of the factors which influence the rate of wear of conventionalhip-joint implants, the most significant are patient weight and activitylevel. Additionally, heat generated by friction in the normal use of theimplant has been shown to cause accelerated creep and wear of thepolyethylene cup. Furthermore, there is a correlation between thefrictional moment which transfers torque loading to the cup and thefrictional coefficient between the femoral head and the surface of theacetabular cup against which the head articulates. Cup torque has beenassociated with cup loosening. Thus, in general, the higher thecoefficient of friction for a given load, the higher the level of torquegenerated. Ceramic bearing surfaces have been shown to producesignificantly lower levels of frictional torque. It is also noteworthythat two of the three commonly used hip-joint systems as indicated aboveinclude a metallic femoral head articulating against an ultra highmolecular weight polyethylene (UHMWPE) liner inside the acetabular cup.UHMWPE, being a polymeric material, is more susceptible to creep athigher temperatures than the commonly used metal alloys or ceramics dueto its relatively lower melting point and is consequently moresusceptible to wear than the alloys or ceramics.

[0008] The original impetus for the inclusion of surfaces such as UHMWPEwas that they would act sacrificially; they would fail slowly and failbefore the harder surface, allowing for an overall extension of theuseful life of the device. Additionally, polyethylene was thought toabsorb shock much better than harder surfaces, thereby simulating theeffect of real cartilage. While the advance in the art which wasrealized by the use of oxidized zirconium surfaces articulating againstUHMWPE surfaces was a lessening of wear and cup loosening between thesurface of the metallic component and the UHMWPE, the problem was notcompletely eliminated. Thus, the instant invention represents anotheradvancement in the art, namely, a further improvement in wear and asimultaneous significant improvement of the creep problem associatedwith the prior art prostheses comprising polyethylene articulatingagainst harder surfaces.

[0009] It has also been found that metal prostheses are not completelyinert in the body. Body fluids act upon the metals causing them toslowly corrode by an ionization process thereby releasing metal ionsinto the body. Metal ion release from the prosthesis is also related tothe articulation and rate of wear of load bearing surfaces because, asmay be expected, when a metallic femoral head, for instance, isarticulated against UHMWPE, the passive oxide film which forms on thefemoral head is constantly removed. The repassivation process constantlyreleases metal ions during this process. Furthermore, the presence ofthird-body wear (cement or bone debris) accelerates this process andmicro-fretted metal particles can increase friction. Consequently, theUHMWPE liner inside the acetabular cup, against which the femoral headarticulates, is subjected to accelerated levels of creep, wear, andtorque. A reduction in these deleterious effects will also improve theproblem of metal ion release.

[0010] A number of attempts to correct these problems were the subjectof much of the early work in this area. U.S. Pat. No. 4,145,764 toSuzuki taught a metal prosthesis plasma sprayed with a bonding agentwhich is in turn covered with a porous ceramic coating which would allowthe in-growth of bone spicules into the pores. However, the Suzukipatent did not address the issue of friction or wear of orthopedicimplant bearing surfaces but confined itself to the single issue of thebiocompatibility of metal prostheses and did not address the issue ofdimensional changes that occur when applying such a coating. U.S. Pat.No. 3,677,795 to Bokros is directed to the application of a carbidecoating over a metallic prosthetic device. The method is said to producea prosthetic device which has “excellent compatibility with body tissueand is non-thrombogenic”. However, Bokros does not address the issues offriction, heating, creep and wear of orthopedic implant bearingsurfaces, or changes induced in the mechanical properties of theunderlying metal due to this high-temperature treatment.

[0011] The aforementioned failings of the prior art were addressed inpart by Davidson in U.S. Pat. No. 5,037,438. In the '438 patent,Davidson teaches a zirconium or zirconium-containing metal alloyprosthesis coated via in-situ oxidation with a surface of blue-black orblack oxidized zirconium which articulates against an organic polymer orpolymer-based composite. The oxidized zirconium coating provides theprosthesis with a thin, dense, low friction, wear resistant,biocompatible surface ideally suited for use on articulating surfaces ofjoint prostheses wherein a surface or surfaces of the joint articulates,translates or rotates against mating joint surfaces. The oxidizedzirconium coating described the '438 patent may therefore be usefullyemployed on the femoral heads or inside surfaces of acetabular cups ofhip-joint implants or on the articulating surfaces of other types ofprostheses, such as knee joints. Notably, the oxidized zirconium coatingof the '438 patent was a specific type of oxidized zirconium. Oxidizedzirconium presents itself in many forms, among them are white, beige,and blue-black. The white variety is particularly disfavored in thepresent application, as it tends to separate and break off of thesubstrate. Conventional oxidized zirconium surfaces formed, for example,by simple air oxidation will not be of the blue-black or black varietyand will not possess the superior properties of the same which arerecited in the '438 patent. The most important of these properties highhardness, low friction that results from the presence of the surfaceoxide.

[0012] The specific blue-black or black oxidized zirconium coatings ofthe '438 patent were known in the art of mechanical bearings, havingbeen described in U.S. Pat. No. 2,987,352, which teaches a 700-1100° F.oxidation method to produce the specific blue-black or blue oxidizedzirconium coating. A later issuing patent to Haygarth (U.S. Pat. No.4,671,824) teaches an alternative, salt-bath method to produce thedesired coating. The blue-black or black oxidized zirconium of theinstant invention possessing the necessary properties is primarilymonoclinic crystal structure. This has been characterized by Hunter etal. (Hunter, G, et al., Mat. Res. Symp. Proc., (1999), 550, 337).

[0013] The introduction of XLPE as a counter bearing surface inarticulating prostheses was another unrelated attempt to address theproblem of the relatively short service life of the UHMWPE component.Cross-linking of UHMWPE forms covalent bonds between polymer chainswhich retard the process of wear through the internal reinforcement ofthe individual polymer chains. However, XLPE is not without problems.The advantages of XLPE over other forms of polyethylene diminish as theroughness of the counter bearing surface increases and the operatingtemperature of device increases. Thus, a counter bearing surface, whichpossesses properties that prevent or improve the aforementionedconditions, would accentuate the advantages of XLPE over conventionalforms of polyethylene. Additionally, there is anecdotal evidence thatthe improvement of wear characteristics comes at a cost of greatersusceptibility to creep, particularly at elevated temperatures.

[0014] XLPE devices have exhibited other deficiencies, with which theprior art has largely been concerned about. Free radicals formed duringirradiation, however, can exist indefinitely if termination bycross-linking or other forms of recombination do not occur. Furthermore,reacted intermediates are continuously formed and decayed. Exposure ofthese free radical species at any time (e.g., during irradiation,shelf-aging, or in vivo aging) to molecular oxygen or any other reactiveoxidizing agent can result in their oxidation. Extensive oxidation leadsto a reduction in molecular weight, and subsequent changes in physicalproperties, including wear resistance. Many attempts have been made toimprove the characteristics of XLPE. These attempts include radiationinduced cross linked polyethylene (See U.S. Pat. Nos. 5,728,748;5,650,485; 5,543,471; 5,414,049; and 5,449,745 to Howmedica; Johnson &Johnson's EP 0737481 A1, see also Hamilton, J. V. et al., ScientificExhibit, 64^(th) AAOS Meeting, February 1997; Hamilton, J. V. et al.,Trans 43^(rd) ORS, 782, 1997; Biomet's World Patent Application No.97/29787; see also Oonishi, H. et al., Radiat. Phys. Chem., 39(6), 495,1992; Oonishi, H. et al., Mat. Sci: Materials in Medicine, 7, 753-63,1966, Oonishi, H. et al., J. Mat. Sci: Materials in Medicine, 8, 11-18,1997; U.S. Pat. No. 5,879,400; World Patent Application WO 98/01085;U.S. Pat. No. 6,165,220; EP 0729981 A1; U.S. Pat. No. 6,017,975; andU.S. Pat. No. 6,228,900, for chemical cross-linking of polyethylene (SeeEP 0722973 A1)).

[0015] In the present invention, the improvement in the performance ofXLPE is realized not through improvements in the XLPE compositionitself, but rather through the use of oxidized zirconium or otherabrasion resistant surface as a counter bearing surface against whichthe XLPE component articulates. The advantages of the instant inventionwill be the preservation of the desirable properties of XLPE with asimultaneous elimination of some of its negative properties. The '438patent did not contemplate and nowhere does it teach, the use of theoxidized zirconium surfaces directly contacting surfaces of XLPE. Theinventors have discovered that the unique properties of oxidizedzirconium accentuate the inherent advantages of XLPE as a counterbearing surface. The superior strength and hardness, low friction, wearresistance, thermal conductivity, and biocompatibility characteristicsof the blue-black or black oxidized zirconium is sufficient in itself toconsiderably slow and possibly prevent the degradative wear processes towhich the prosthetic devices of the prior art have been subject. Anunapparent synergy is realized because the unique properties of oxidizedzirconium serve to improve the performance of XLPE as a counter bearingsurface. These unexpected advantages are also present, to a lesserdegree, when other abrasion resistant surfaces are used.

[0016] The invention is directed to forming prosthetic devices ofoxidized zirconium-on-XLPE, which represents a special species ofoxidized zirconium-on-polymer devices, exhibiting even longer overalluseful service life relative to conventional prosthesesmaterials-on-UHMWPE. This is due not only to the advantages which inureupon the substitution of oxidized zirconium for conventional prosthesismaterials, but also from the synergistic improvement in XLPE performancethat is seen when XLPE articulates against oxidized zirconium. Theinvention is not limited to prostheses formed of zirconium or zirconiumalloy. The prostheses substrate may itself be a composite material, onlyrequiring that zirconium or zirconium alloy be present in the substratelayer immediately adjacent to the surface upon which the oxidizedzirconium coating is to be formed.

SUMMARY OF THE INVENTION

[0017] As used herein, “a” or “an” may mean one or more. As used hereinin the claim(s), when used in conjunction with the word “comprising”,the words “a” or “an” may mean one or more than one. As used herein“another” may mean at least a second or more.

[0018] As used herein, “abrasion resistant surface” is defined as amaterial surface having a greater hardness than the underlyingsubstrate. When used in reference to the underlying substrate material,it is synonymous with “surface hardened”. An oxidized zirconium surfaceis one example of an abrasion resistant surface. Others include, but arenot limited to substrates coated with ceramic materials.

[0019] As used herein, “base material” is defined as the material uponwhich a layer of oxidized zirconium is to be formed. It may behomogeneous, consisting of a single phase material, or it may beheterogeneous, consisting of a composite material of one or moresubstrate layers.

[0020] As used herein, the term “contacting surface” refers to any twosurfaces of the prosthetic device or medical implant that contact oneanother in either a load bearing (articulating) or non-load bearing(non-articulating) manner.

[0021] As used herein, “substrate layer” is defined as a distinctchemical region or domain within the base material. A substrate layermay or may not be comprised of zirconium or zirconium alloy.

[0022] As used herein, “zirconium alloy” is defined as any metal alloycontaining zirconium in any amount greater than zero. Thus, an alloy inwhich zirconium is a minor constituent is considered a “zirconium alloy”herein.

[0023] The following discussion contains illustration and examples ofpreferred embodiments for practicing the present invention. However,they are not limiting examples. Other examples and methods are possiblein practicing the present invention and would be apparent to one ofskill in the art upon reading the present disclosure.

[0024] In one embodiment of the present invention, a knee prosthesis forimplantation in a patient comprises a prosthesis body formed ofzirconium or zirconium alloy. The prosthesis body has an implant portionfor inserting into the body tissue of a patient, a bearing surfacehaving at least one condyle on the prosthesis body, a tibial componentcomprised of cross-linked polyethylene and having a surface adapted tocooperate with the bearing surface, and a thin coating of blue-black orblack oxidized zirconium directly on the bearing surface of the condyleportion.

[0025] In specific embodiments of a knee prosthesis, the thin blue-blackor black oxidized zirconium coating is from about 1 to about 20 micronsthick. Preferably, the blue-black or black oxidized zirconium coating isfrom about 1 to about 5 microns thick. In a further embodiment, the kneeprosthesis has an implant portion of the prosthesis body that has, atleast in part, an irregular surface structure adapted to accommodatetissue in-growth on a portion of the prosthesis body. In a specificembodiment of the irregular surface structure prosthesis, the irregularsurface structure can have zirconium or zirconium alloy beads attachedto the outer surface of the prosthesis body, with at least a portion ofthe surface of the beads being oxidized to blue-black or black oxidizedzirconium. In another embodiment of the irregular surface structureprosthesis, the irregular surface structure can have zirconium orzirconium alloy wire mesh connected to the outer surface of theprosthesis body, wherein at least a portion of the surface of the meshis oxidized to blue-black or black oxidized zirconium.

[0026] In another embodiment of the invention, a hip prosthesis forimplantation in a patient includes a hip prosthesis body forimplantation into a femur which has a head portion formed at least inpart of zirconium or zirconium alloy, a bearing surface on the headportion of the prosthesis body, and an acetabular cup having an innersurface comprising cross-linked polyethylene. A cooperating surface isadapted to engage and articulate with the bearing surface on the headportion. A thin coating of blue-black or black oxidized zirconium isformed directly on the exposed bearing surface of the head portion.

[0027] In a specific embodiment of the hip prosthesis, the thinblue-black or black oxidized zirconium coating is from about 1 to about20 microns thick. Preferably, the thin blue-black or black oxidizedzirconium coating is from about 1 to about 5 microns thick. In anotherembodiment, the prosthesis body further includes an irregular surfacestructure adapted to accommodate tissue in-growth on a portion of theprosthesis body. In a specific embodiment of the irregular surfacestructure prosthesis, the irregular surface structure is formed ofzirconium or zirconium alloy beads connected to the outer surface of theprosthesis body. At least a portion of the surface of the beads isoxidized to blue-black or black oxidized zirconium. In yet anotherembodiment of the irregular surface structure prosthesis, the irregularsurface structure is formed of zirconium or zirconium alloy wire meshconnected to the outer surface of the prosthesis body. At least aportion of the surface of the mesh is oxidized to blue-black or blackoxidized zirconium.

[0028] In another embodiment of the invention, a prosthesis forimplantation in a patient includes a prosthesis body formed at least inpart of zirconium or zirconium alloy. The prosthesis body includes animplant portion for insertion into the body tissue of the patient, abearing surface on the prosthesis body, the bearing surface being sizedand shaped to engage and articulate with a second bearing surface onanother prosthesis portion. The second bearing surface is formed of across-linked polyethylene. The prosthesis also includes a coating ofblue-black or black oxidized zirconium from about 1 to about 5 micronsin thickness on the bearing surface.

[0029] In a specific embodiment, the prosthesis body is a hip jointhaving a head portion as a bearing surface and another prosthesisportion is an acetabular cup, the head portion being adapted tocooperate with the inner surface of the acetabular cup, and the innersurface of the acetabular cup is also the second bearing surface. Inanother embodiment the prosthesis body is a knee joint and the bearingsurface of the prosthesis body includes at least one condyle, andanother prosthesis portion including a tibial component, with at leastone condyle being adapted to cooperate with the tibial component. In aspecific embodiment, the prosthesis body has an irregular surfacestructure adapted to accommodate tissue in-growth on at least a portionof the prosthesis body. In another embodiment of the irregular surfacestructure prosthesis, the irregular surface structure is formed ofzirconium or zirconium alloy beads connected to the outer surface of theprosthesis body, where at least a portion of the surface of the beads isoxidized to blue-black or black oxidized zirconium. In a specificembodiment, the irregular surface structure prosthesis has an irregularsurface structure formed of zirconium or zirconium alloy wire meshconnected to the outer surface of the prosthesis body, wherein at leasta portion of the surface of the mesh is oxidized to blue-black or blackoxidized zirconium.

[0030] In another embodiment, the invention is directed towards aprosthesis for implantation in a patient which includes a prosthesisbody formed at least in part of zirconium or zirconium alloy, theprosthesis body includes an implant portion for inserting into the bodytissue of the patient, a bearing surface on the prosthesis body, acounter-bearing surface formed of cross-linked polyethylene and adaptedto cooperate with the bearing surface and, a thin coating of blue-blackor black oxidized zirconium directly on the bearing surface.

[0031] In a specific embodiment, the thin blue-black or black oxidizedzirconium coating is from about 1 to about 20 microns thick. Preferably,the thin blue-black or black oxidized zirconium coating is from about 1to about 5 microns thick. In another embodiment, the implant portion ofthe prosthesis body further includes an irregular surface structureadapted to accommodate tissue in-growth on a portion of the prosthesisbody. In a specific embodiment of the irregular surface structureprosthesis, the irregular surface structure is formed at least in partof zirconium or zirconium alloy beads attached to the outer surface ofthe prosthesis body, with at least a portion of the surface of the beadsbeing oxidized to blue-black or black oxidized zirconium. In yet anotherspecific embodiment of the irregular surface structure, the irregularsurface structure is formed of zirconium or zirconium alloy wire meshconnected to the outer surface of the prosthesis body, with at least aportion of the surface of the mesh being oxidized to blue-black or blackoxidized zirconium.

[0032] In another embodiment, a prosthesis for implantation in apatient, includes a prosthesis body comprising a substrate material. Theprosthesis body includes an implant portion for inserting into the bodytissue of the patient, a bearing surface on the prosthesis body formedof any abrasion resistant surface, and a counter-bearing surface formedof cross-linked polyethylene and adapted to cooperate with the bearingsurface. In a specific embodiment, the implant portion of the prosthesisbody further includes an irregular surface structure adapted toaccommodate tissue in-growth on a portion of the prosthesis body. In aspecific embodiment of the irregular surface structure prosthesis, theirregular surface structure has beads attached to the outer surface ofthe prosthesis body, with at least a portion of the surface of the beadsis oxidized to blue-black or black oxidized zirconium. In anotherspecific embodiment of the irregular surface structure prosthesis, theirregular surface structure is formed wire mesh connected to the outersurface of the prosthesis body, with at least a portion of the surfaceof the mesh is oxidized to blue-black or black oxidized zirconium.

DESCRIPTION OF THE DRAWINGS

[0033]FIG. 1 is a schematic diagram depicting a hip joint prosthesis inposition.

[0034]FIG. 2 is a schematic diagram showing a typical hip jointprosthesis.

[0035]FIG. 3 is a schematic diagram of a knee joint prosthesis in place.

[0036]FIG. 4 is a schematic diagram of the parts of a typical kneejoint.

DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention provides low friction, wear and creepresistant, articulating, interfacial bearing surfaces for prostheticdevices. Preferably, the invention provides for a prosthetic device inwhich one contacting surface is formed of blue-black or black oxidizedzirconium and another contacting surface is formed of XLPE.Alternatively, one contacting surface is formed of an abrasion resistantmaterial and another contacting surface is XLPE.

[0038] Oxidized zirconium presents itself in many forms, among them arewhite, beige, and blue-black. The white variety is particularlydisfavored in the present application, as it tends to separate and breakoff of the substrate readily. Conventional oxidized zirconium surfacesformed, for example, by simple air oxidation will not be of theblue-black or black variety.

[0039] The blue-black or black oxidized zirconium of the instantinvention possessing the necessary properties is primarily monocliniccrystal structure and may include tetragonal zirconia. Itsmicrostructure has been characterized by Hunter et. al. The specificblue-black or black oxidized zirconium coatings used herein were knownin the art of mechanical bearings, having been originally taught byWatson in U.S. Pat. No. 2,987,352. Davidson, in U.S. Pat. No. 5,037,428,first taught the application of this form of oxidized zirconium toprosthetic devices.

[0040] Importantly, the base materials upon which the oxidized zirconiumcoating is formed need not be fabricated totally of zirconium orzirconium alloy. The only requirement for the formation of an oxidizedzirconium surface is zirconium or zirconium alloy material in thesurface before the formation of the oxidized zirconium layer. Uponformation of the oxidized zirconium surface the former surface becomesthe first substrate layer. For example, composite materials are possibleembodiments of the present invention. This allows one to realizestrength and weight advantages of prostheses having one or moredifferent core materials. For example, a synthetic core material havinghigh strength and being of light weight and having a zirconium orzirconium alloy veneer or outer layer is envisioned by the presentinvention. The interior of the base material may be of one or more thanone material, allowing for varying degrees of heterogeneity in theoverall fabrication, depending upon the application. Alternatively, alesser degree of heterogeneity is possible. For example, a base materialbeing relatively rich in zirconium or zirconium alloy at the surface andexhibiting a continuously decreasing zirconium content as a depth intothe substrate increases is another possibility. In this latter example,no abrupt macroscopic phase boundary exists within the base material.These examples are meant to be merely illustrative and not exhaustive;variations in the possible embodiments which would be apparent to one ofordinary skill in the art upon a reading of this disclosure are part ofthe present invention.

[0041] The XLPE useful in the present invention is polyethylenecross-linked by any means, irradiative or chemical being illustrativeexamples which are generally known in the art. Illustrative examples ofthe prosthetic devices for which the contact interfaces disclosedhererin find use are shown in the schematic diagrams, FIGS. 1-4.

[0042] A typical hip joint assembly is shown in situ in FIG. 1 and FIG.2. The hip joint stem 2 fits into the femur while the femoral head 6 ofthe prosthesis fits into and articulates against the inner lining 8 ofan acetabular cup 10 which in turn is affixed to the pelvis as shown inFIG. 1. A porous metal bead or wire mesh coating 12 may be incorporatedto allow stabilization of the implant by in-growth of surrounding tissueinto the porous coating. Similarly, such a coating can also be appliedto the acetabular component. The femoral head 6 may be an integral partof the hip joint stem 2 or may be a separate component mounted upon aconical taper at the end of the neck 4 of the hip joint prosthesis. Thisallows the fabrication of a prosthesis having a metallic stem and neckbut a femoral head of a different material.

[0043] This method of construction is often desirable because the use ofcomposite materials allows for the localized optimization of a varietyof parameters such as weight, strength and wear resistance. Regardlessof the materials, however, the femoral head articulates against theinner surface of the acetabular cup thereby causing wear and, in thelong term, this may necessitate prosthesis replacement. This isespecially the case where the femoral head is of metal and theacetabular cup is lined with an organic polymer or composite thereof.While these polymeric surfaces provide good, relatively low frictionsurfaces and are biocompatible, they are, as explained above, subject towear and accelerated creep due to the frictional heat and torque towhich they are subjected during ordinary use.

[0044] In the present invention, the inner lining 8 is preferably formedof XLPE and the femoral head 6 is coated with a surface of blue-black orblack oxidized zirconium. Preferably, the oxidized zirconium surfacecoating is 1-20 μm thick, but may be outside this range. It mayalternatively be from about 1-5 μm thick. The wire mesh or metal beadsurface 12 may be at least partially coated with a surface of blue-blackor black oxidized zirconium to promote in-growth of tissue or bone intothe device, thereby stabilizing its position.

[0045] A typical knee joint prosthesis is shown in situ in FIG. 3 andFIG. 4. The knee joint includes a femoral component 20 and a tibialcomponent 30. The femoral component includes condyles 22 which providethe articulating surface of the femoral component and pegs 24 foraffixing the femoral component to the femur. The tibial component 30includes a tibial base 32 with a peg 34 for mounting the tibial baseonto the tibia. A tibial platform 36 is mounted atop the tibial base 32and is supplied with grooves 38 similar to the shape of the condyles 22.The bottom surfaces of the condyles 26 contact the tibial platform'sgrooves 38 so that the condyles articulate within these grooves againstthe tibial platform. In the present invention, the condyles 22 arecoated with a surface of blue-black or black oxidized zirconium and thegrooves 38 on the tibial platform 36 are comprised of XLPE. Part or allof the remainder of the femoral component 20 and a tibial component 30may have a surface coating of blue-black or black oxidized zirconium.Preferably, the surface coatings are 1-20 μm thick, but may be outsidethis range. They may alternatively be from about 1-5 μm thick. As in thecase of the hip joint, porous bead or wire mesh coatings can also beapplied to either the tibial or femoral components of the knee or both.These porous bead or wire mesh coatings of the knee prosthesis may alsobe at least partially coated with a surface of blue-black or blackoxidized zirconium to promote in-growth of tissue or bone into thedevice, thereby stabilizing its position.

[0046] The base (i.e., substrate) zirconium containing metal alloys arecast or machined by conventional methods to the shape and size desiredto obtain a prosthesis substrate. The substrate is then subjected toprocess conditions which cause the natural (in situ) formation of atightly adhered, diffusion-bonded coating of oxidized zirconium on itssurface. The process conditions include, for instance, air, steam, orwater oxidation or oxidation in a salt bath. These processes ideallyprovide a thin, hard, dense, blue-black or black, low-frictionwear-resistant oxidized zirconium film or coating of thickness typicallyon the order of several microns (10⁻⁶ meters) on the surface of theprosthesis substrate. Below this coating, diffused oxygen from theoxidation process increases the hardness and strength of the underlyingsubstrate metal.

[0047] Importantly, the base materials need not be formed totally ofzirconium or zirconium alloy. The only requirement for the formation ofan oxidized zirconium surface is that zirconium or zirconium alloymaterial comprises the surface layer of the base material before theformation of the oxidized zirconium layer. For example, compositematerials are possible embodiments of the present invention. This allowsone to realize strength and weight advantages of prostheses having oneor more different core materials. For example, a synthetic core materialhaving high strength and being of light weight surrounded by a zirconiumor zirconium alloy veneer or outer layer is a possible embodiment of thepresent invention. One or more substrata may comprise the base materialand these substrata may be comprised of any suitable material which mayor may not include zirconium or zirconium alloy. Alternatively, the basematerial may consist of a continuous phase in which the concentration ofzirconium or zirconium alloy varies with depth into the base material.For example, the surface at which the coating of oxidized zirconium isformed may be relatively rich in zirconium, with this concentrationfalling off as the distance into the base material (i.e., away from thesurface) increases. Variations on the general embodiment are within thescope of the present invention in light of the disclosure.

[0048] Representative methods for the formation of the surface coatingof blue-black or black oxidized zirconium have been described previouslyin U.S. Pat. Nos. 2,987,352 to Watson and 5,037,428 to Davidson, whichare incorporated by reference as though fully set forth herein. Acoating thickness of 1 to 5 μm is preferred. Conditions useful for thefabrication of surfaces of varying thickness are described in U.S. Pat.No. 5,037,428. Methods for controlling the uniformity of the oxidizedzirconium coating are described in the commonly-assigned copendingapplication Ser. No. 09/381,217, which is incorporated by reference asthough fully set forth herein.

[0049] Oxygen, niobium, and titanium are common alloying elements in thealloy with oftentimes the presence of hafnium. While such zirconiumcontaining alloys may be custom formulated by conventional methods knownin the art of metallurgy, a number of suitable alloys are commerciallyavailable. Illustrative examples of these commercial alloys includeamong others Zircadyne 705, Zircadyne 702, and Zircalloy.

[0050] These diffusion-bonded, low friction, highly wear resistantoxidized zirconium coatings are applied to one of the contactingsurfaces of orthopedic implants subject to conditions of wear. Suchsurfaces include the articulating surfaces of knee joints, elbows andhip joints. In the case of hip joints, the femoral head and stem aretypically fabricated of metal alloys while the acetabular cup may befabricated from ceramics. The only requirement is that the surface ofthe base material of the contacting surface of the device must be ofzirconium or zirconium alloy such that upon surface oxidation under theappropriate conditions, measurable surface concentrations of oxidizedzirconium are formed. Substrate layers extending into the interior ofthe base material may consist of one or more materials, any, or, or noneof which may be of zirconium or zirconium alloy.

[0051] The wear advantages of XLPE over conventional polyethylene arediminished under certain conditions. For instance as the roughness ofthe counter bearing surface increases, XLPE approaches conventionalpolyethylene in its wear resistance (See, e.g., McKellop et al., “Wearof Gamma-Crosslinked Polyethylene Acetabular Cups Against RoughenedFemoral Balls”, Clin. Orthop., (1999) 369, pp. 73-82). The wearadvantage of XLPE over conventional polyethylene enhanced by using a lowroughness, abrasion resistant articulating surfaces against the XLPE.

[0052] There is anecdotal evidence that cross-linked polyethylene ismore prone to creep than conventional polyethylene. Thus, there is apotential trade-off to be made in gaining the superior wearcharacteristics of XLPE over conventional PE in medical implants.However, the articulating bearing surfaces unexpectedly diminished thisdisadvantage. Creep is a relatively slow plastic deformation processwhich is more prominent at higher temperatures as a material moreclosely approaches its melting point. (See “Metallurgy Fundamentals”, D.A. Brandt and J. C. Warner, Goodheart-Wilcox Pub. Co., Inc., p. 57,1999). A contacting surface that is a thermal conductor will diminishthe creep resistance advantage of conventional polyethylene over XLPEthrough its ability to pass heat and avoid a localized rise intemperature. As the temperature of either polymer rises, its ability towithstand protracted stress-strain loads decreases, such that prosthesesmade of either conventional polyethylene or XLPE are subject to failure.At temperatures significantly lower than the melting point, the twopolymers more closely approach one another in their creep resistance.

[0053] Oxidized zirconium, is unique with respect to the excellentthermal conductivity it possesses relative to other conventionalprosthetic materials. It combines excellent surface roughnesscharacteristics with very high thermal conductivity. In this way, itpossesses the relevant beneficial characteristics (in terms of XLPE) ofmetal and ceramics while avoiding the relevant disadvantages of theformer and outperforming the latter. Thus, while all abrasion resistantsurfaces such as zirconia and alumina would enjoy the benefit of reducedwear of XLPE and at least some improvement in creep resistance, oxidizedzirconium would enjoy both a significant reduced wear and a significantreduced creep susceptibility. Table 1 below schematically illustratesthese characteristics. TABLE 1 Relative Performance of VariousProstheses Surfaces with Respect to Various Characteristics and OverallDegradative Effects on Counter Bearing Surfaces OXIDIZED ZIRCONIUMZIRCONIA ALUMINA METAL Surface 3 2 2 1 Roughness Strength 3 1 0 3 Wear(of itself) 2 2 3 0 Thermal 3 0 3 3 Conductivity Wear and 2 2 2 0 Creepof Poly- ethylene Wear and 3 2 2 0 Creep of XLPE

[0054] The prostheses of the instant invention exhibit design advantagesin a variety of applications. Importantly, the wear resistancecharacteristics of XLPE, already superior relative to UHMWPE is fullyrealized while the creep characteristics are improved when XLPEarticulates against abrasion resistant surfaces. These improvements arefurther enhanced when XLPE articulates against oxidized zirconium,allowing for the polymer lining to be constructed of lower thicknessthan in conventional prostheses. The superiority of oxidized zirconiumcounter bearing surfaces in terms of low friction and high thermalconductivity allows for polymer linings that are thinner still. Thesynergistic result is a potentially smaller prosthesis having weightadvantages over conventional prostheses, which requires less boneresection, greater conformity and contact area, and exhibits reductionsin contact stress, an increase in range of motion, and a decreasedchance of dislocation. A smaller amount of polymer material is expectedto result in a lower likelihood of osteolysis (resulting from a lesserlikelihood of material migration) while having the same or greatercomponent service life. Decreases in mass of the prosthesis is expectedto be of greater significance in hip applications which are typicallylarger than knee prostheses and have a smaller range of motion.

[0055] The present invention is also useful in knee prostheses. Kneeprostheses typically are of two varieties: mobile bearing knees (MBK)and fixed bearing knees (FBK). Referring to FIGS. 3 and 4, the MBK ischaracterized by having its articulating tibial component 30 movablerelative to the tibial platform 36. This results in wear and creep onboth the top and bottom of the tibial component 30. In contrast, thisinterface in the FBK is fixed and the only movement is between the topof the tibial component and the condyles 22. As a result, the MBK has agreater range of motion but suffers from greater wear and creep than anFBK. XLPE generally offers greater wear and comparable creep resistanceover UHMWPE when articulating against abrasion resistant surfaces. Thischaracteristic is enhanced where the tibial tray is formed of oxidizedzirconium. This same advantage is also seen in FBK prostheses, but islimited to the articulating interface between the tibial component 30and the condyles 22.

[0056] Zirconium or zirconium alloy can also be used to provide a porousbead or wire mesh surface to which surrounding bone or other tissue mayintegrate to stabilize the oxidized zirconium-on-oxidized zirconiumprosthesis. These porous coatings can be treated simultaneously by theoxidation treatment in a manner similar to the oxidation of the baseprosthesis for the elimination or reduction of metal ion release.Furthermore, zirconium or zirconium alloy can also be used as a surfacelayer applied over conventional implant materials prior to in situoxidation and formation of the oxidized zirconium coating.

[0057] Although the invention has been described with reference to itspreferred embodiments, those of ordinary skill in the art may, uponreading this disclosure, appreciate changes and modifications which maybe made and which do not depart from the scope and spirit of theinvention as described above or claimed hereafter.

References

[0058] All patents and publications mentioned in the specification areindicative of the level of those skilled in the art to which theinvention pertains. All patents and publications are herein incorporatedby reference to the same extent as if each individual publication wasspecifically and individually indicated to be incorporated by reference.U.S. Pat. No. 5,037,438 8/1991 Davidson U.S. Pat. No. 4,145,746 3/1979Suzuki et al. U.S. Pat. No. 3,677,795 7/1972 Bokros et al. U.S. Pat. No.2,987,352 2/1958 Watson U.S. Pat. No. 4,671,824 6/1987 Haygarth U.S.App. S/N 09/381,217 filed 11/1999 Hunter et al.

[0059] Hunter, G. et al., Mat. Res. Soc. Symp. Proc., 1999, 550, 337.

[0060] McKellop et al., “Wear of Gamma-Crosslinked PolyethyleneAcetabular Cups Against Roughened Femoral Balls”, Clin. Orthop., (1999)369, pp. 73-82.

[0061] Hunter et al., “Abrasive Wear of Oxidized Zr-2.5Nb, CoCrMo, andTi-6Al-4V Against Bone Cement”, 6^(th) World Biomaterials Cong. Trans.,Society for Biomaterials, Minneapolis, Minn., (2000), p. 835

What is claimed is:
 1. A prosthesis for implantation in a patient,comprising: (a) a prosthesis body comprising zirconium or zirconiumalloy, said prosthesis body comprising an implant portion for insertinginto the body tissue of the patient; (b) a bearing surface comprising atleast one condyle on the prosthesis body; (c) a tibial componentcomprising cross-linked polyethylene having a surface adapted tocooperate with the bearing surface; and, (d) a thin coating ofblue-black or black oxidized zirconium directly on the bearing surfaceof the condyle portion.
 2. The prosthesis of claim 1 wherein said thinblue-black or black oxidized zirconium coating is from about 1 to about20 microns thick.
 3. The prosthesis of claim 1 wherein said thinblue-black or black oxidized zirconium coating is from about 1 to about5 microns thick.
 4. The prosthesis of claim 1 wherein the implantportion of the prosthesis body further comprises an irregular surfacestructure adapted to accommodate tissue in-growth on a portion of theprosthesis body.
 5. The prosthesis of claim 1 wherein the irregularsurface structure comprises zirconium or zirconium alloy beads attachedto the outer surface of the prosthesis body, wherein at least a portionof the surface of the beads is oxidized to blue-black or black oxidizedzirconium.
 6. The prosthesis of claim 1 wherein the irregular surfacestructure comprises zirconium or zirconium alloy wire mesh connected tothe outer surface of the prosthesis body, wherein at least a portion ofthe surface of the mesh is oxidized to blue-black or black oxidizedzirconium.
 7. The prosthesis of claim 1 wherein the prosthesis bodyfurther comprises at least one substrate layer.
 8. The prosthesis ofclaim 1 wherein the prosthesis body further comprises at least onesubstrate layer having a depth-dependent variable concentration ofzirconium.
 9. A prosthesis for implantation in a patient, comprising:(a) a hip prosthesis body for implantation into a femur comprising ahead portion, said head portion comprising zirconium or zirconium alloy;(b) a bearing surface on the head portion of the prosthesis body; (c) anacetabular cup having an inner surface comprising cross-linkedpolyethylene, said inner surface being adapted to cooperate with thebearing surface on the head portion; and, (d) a thin coating ofblue-black or black oxidized zirconium directly on the bearing surfaceof the head portion.
 10. The prosthesis of claim 9 wherein said thinblue-black or black oxidized zirconium coating is from about 1 to about20 microns thick.
 11. The prosthesis of claim 9 wherein said thinblue-black or black oxidized zirconium coating is from about 1 to about5 microns thick.
 12. The prosthesis of claim 9 wherein the prosthesisbody further comprises an irregular surface structure adapted toaccommodate tissue in-growth on a portion of the prosthesis body. 13.The prosthesis of claim 9 wherein the irregular surface structurecomprises zirconium or zirconium alloy beads connected to the outersurface of the prosthesis body, wherein at least a portion of thesurface of the beads is oxidized to blue-black or black oxidizedzirconium.
 14. The prosthesis of claim 9 wherein the irregular surfacestructure comprises zirconium or zirconium alloy wire mesh connected tothe outer surface of the prosthesis body, wherein at least a portion ofthe surface of the mesh is oxidized to blue-black or black oxidizedzirconium.
 15. The prosthesis of claim 9 wherein the hip prosthesis bodyfurther comprises at least one substrate layer.
 16. The prosthesis ofclaim 9 wherein the hip prosthesis body further comprises at least onesubstrate layer having a depth-dependent variable concentration ofzirconium.
 17. A prosthesis for implantation in a patient, comprising:(a) a prosthesis body comprising zirconium or zirconium alloy, saidprosthesis body comprising an implant portion for insertion into thebody tissue of the patient; (b) a bearing surface on the prosthesisbody, the bearing surface being sized and shaped to engage or cooperatewith a second bearing surface on another prosthesis portion, said secondbearing surface comprising cross-linked polyethylene; and, (c) a coatingof blue-black or black oxidized zirconium from about 1 to about 5microns in thickness on the bearing surface of the prosthesis.
 18. Theprosthesis of claim 17 wherein the prosthesis body is a hip joint havinga head portion as a bearing surface and wherein said another prosthesisportion is an acetabular cup, said head portion being adapted tocooperate with the inner surface of the acetabular cup, wherein saidinner surface of said acetabular cup is also said second bearingsurface.
 19. The prosthesis of claim 17 wherein the prosthesis body is aknee joint and the bearing surface of the prosthesis body comprises atleast one condyle, and wherein said another prosthesis portion comprisesa tibial component, said at least one condyle being adapted to cooperatewith said tibial component.
 20. The prosthesis of claim 17 wherein theprosthesis body further comprises an irregular surface structure adaptedto accommodate tissue in-growth on at least a portion of the prosthesisbody.
 21. The prosthesis of claim 17 wherein the irregular surfacestructure comprises zirconium or zirconium alloy beads connected to theouter surface of the prosthesis body, wherein at least a portion of thesurface of the beads is oxidized to blue-black or black oxidizedzirconium.
 22. The prosthesis of claim 17 wherein the irregular surfacestructure comprises zirconium or zirconium alloy wire mesh connected tothe outer surface of the prosthesis body, wherein at least a portion ofthe surface of the mesh is oxidized to blue-black or black oxidizedzirconium.
 23. The prosthesis of claim 17 wherein the prosthesis bodyfurther comprises at least one substrate layer.
 24. The prosthesis ofclaim 17 wherein the prosthesis body further comprises at least onesubstrate layer having a depth-dependent variable concentration ofzirconium.
 25. A prosthesis for implantation in a patient, comprising:(a) a prosthesis body comprising zirconium or zirconium alloy, saidprosthesis body comprising an implant portion for inserting into thebody tissue of the patient; (b) a bearing surface on the prosthesisbody; (c) a counter-bearing surface comprising cross-linked polyethyleneand adapted to cooperate with the bearing surface; and, (d) a thincoating of blue-black or black oxidized zirconium directly on thebearing surface.
 26. The prosthesis of claim 25 wherein said thinblue-black or black oxidized zirconium coating is from about 1 to about20 microns thick.
 27. The prosthesis of claim 25 wherein said thinblue-black or black oxidized zirconium coating is from about 1 to about5 microns thick.
 28. The prosthesis of claim 25 wherein the implantportion of the prosthesis body further comprises an irregular surfacestructure adapted to accommodate tissue in-growth on a portion of theprosthesis body.
 29. The prosthesis of claim 25 wherein the irregularsurface structure comprises zirconium or zirconium alloy beads attachedto the outer surface of the prosthesis body, wherein at least a portionof the surface of the beads is oxidized to blue-black or black oxidizedzirconium.
 30. The prosthesis of claim 25 wherein the irregular surfacestructure comprises zirconium or zirconium alloy wire mesh connected tothe outer surface of the prosthesis body, wherein at least a portion ofthe surface of the mesh is oxidized to blue-black or black oxidizedzirconium.
 31. The prosthesis of claim 25 wherein the prosthesis bodyfurther comprises at least one substrate layer.
 32. The prosthesis ofclaim 25 wherein the prosthesis body further comprises at least onesubstrate layer having a depth-dependent variable concentration ofzirconium.
 33. A prosthesis for implantation in a patient, comprising:(a) a prosthesis body comprising a substrate material, said prosthesisbody comprising an implant portion for inserting into the body tissue ofthe patient; (b) a bearing surface on the prosthesis body comprised ofan abrasion resistant surface; and, (c) a counter-bearing surfacecomprising cross-linked polyethylene and adapted to cooperate with thebearing surface.
 34. The prosthesis of claim 33 wherein the implantportion of the prosthesis body further comprises an irregular surfacestructure adapted to accommodate tissue in-growth on a portion of theprosthesis body.
 35. The prosthesis of claim 33 wherein the irregularsurface structure comprises beads attached to the outer surface of theprosthesis body, wherein at least a portion of the surface of the beadsis oxidized to blue-black or black oxidized zirconium.
 36. Theprosthesis of claim 33 wherein the irregular surface structure compriseswire mesh connected to the outer surface of the prosthesis body, whereinat least a portion of the surface of the mesh is oxidized to blue-blackor black oxidized zirconium.
 37. The prosthesis of claim 33 wherein theprosthesis body further comprises at least one substrate layer.
 38. Theprosthesis of claim 33 wherein the prosthesis body further comprises atleast one substrate layer having a depth-dependent variableconcentration of zirconium.